Forest Drought Response Index (ForDRI): A Case Study for the Western United States

Monitoring drought impacts in forest ecosystems is a complex process because forest ecosystems are composed of different species with heterogeneous structural compositions. Even though forest drought status is a key control on the carbon cycle, very few indices exist to monitor and predict forest drought stress. Forest Drought Response Index (ForDRI) is a newly developed experimental drought-monitoring tool designed to identify forest drought conditions. The goal of ForDRI research is to identify when trees are experiencing sufficient stress to start limiting water loss because such actions in turn limit tree growth and increase susceptibility to a variety of other biotic and abiotic factors. ForDRI was developed by the National Drought Mitigation Center (NDMC) in collaboration with USDAs Climate hubs and Forest Service. ForDRI integrates 12 types of data, including satellite, climate, evaporative demand, ground water, and soil moisture, into a single hybrid index to estimate tree stress. The model uses Principal Component Analysis (PCA) to determine the contribution of each input variable based on its covariance in the historical records (20032017). A 15-year time series of ForDRI maps at a weekly interval were produced for evaluation. Correlation analysis between Bowen ratio data and ForDRI was conducted at ten Ameriflux sites that had experienced intense drought over the western United States. In addition, comparing the climate records with the tree-ring data for the same period can provide valuable information for ForDRI evaluation. Thus, tree ring annual increment data at fifty-eight sites in the western U.S. were compared with ForDRI values at the corresponding sites. In general, the preliminary analysis of the correlations between ForDRI and tree-ring data are very encouraging for forest drought assessment and monitoring. The first results of these evaluations will be presented. The ForDRI tool is expected to improve drought-monitoring efforts, specifically addressing the gap for better monitoring of the drought impacts on forests that may not be captured with current drought monitoring tools.